Means to prevent external plug valve leakage

ABSTRACT

The tapered plug of the valve is provided with two adjustments: (a) three plug alignment set screws which properly position the plug to obtain a tight line seal and proper port alignment with the body ports, and properly align external sealing means with plug external seal surfaces, and (2) a plug end adjustment to micrometrically compress a plug shoulder seal against external leakage along the actuating stem.

United States Patent Smith 1 51 Nov. 28, 1972 [54] NIEANS TO PREVENTEXTERNAL 3,192,942 7/1965 Manor et al. ..25l/2l4 X PLUG VALVE LEAKAGE3,206,163 9/1965 Freed ..251/309 3,235,272 2/1966 Smith ..251/2l4 X [72]Invent Russell Ohm 3,371,676 3/1968 Mullaney ..251/214 x [73] Assignee:Xomox Corporation, Cincinnati, 3,396,940 8/1968 Jones ..25l/335 R Ohio3,406,707 10/1968 Schenck ..25l/214 X [22] Filed: 1971 PrimaryExaminerHenry T. Klinksiek [21] App]. No.: 118,450 Attorney-J. WarrenKinney, Jr.

, [57] ABSTRACT [52] US. Cl. ..137/375, 277/33, 251/309 51 1m. (:1...Fl6k 27/12, F1612 5/02 The tapemi P s of the valve is Provided withtwo [58] Field of Search ..277/33, 58; 137/375, 377, 381; lustmems=(zilthree P allgnmenf whlch 251,214, 309, 314, 317, 335 properlyposltlon the plug to obtain a tight line seal and proper port alignmentwith the body ports, and properly align external sealing means with plugexter- [56] References cued nal seal surfaces, and (2) a plug endadjustment to UNITED STATES PATENTS micrometrically compress a plugshoulder seal against external leakage along the actuating stem.2,487,436 l l/l949 Goehrmg ..25l/309 2,926,884 3/ I960 Clinkenbeard"251/309 X 16 Claims, 1 Drawing Figure l'l Q40 %;38 7 "'36 1o 44 fi 44no MEANS TO PREVENT EXTERNAL PLUG VALVE LEAKAGE BACKGROUND OF THEINVENTION Considerable activity has been directed to the development ofvalves which are lined with various types of non-metallic materialsadapted to withstand the damaging effects of corrosive fluids, highpressures, and various other elements encountered in the control ofpiped liquids and gases. The lining material employed is usually a gasand liquid impervious material such as Teflon or equivalent substancewhich provides an excellent seal under proper conditions of use, whileat the same time offering minimal resistance to valve plug rotation,particularly in the case of larger valves.

Most valves of the kind referred to pose a persistent problem ofexternal leakage at some stage of service in a pipe system. Moreover, itis not unusual for a new factory-tested valve to present an externalleakage problem which defies solution, when placed in actual service.Such occurrences sometimes involve very costly remedial procedures whenthe fault becomes evident after completion of a complex pipe systeminstallation.

A portion of the relevant prior art is exemplified in the patents to: J.B. Freed, U.S. Pat. No. 2,913,219, dated Nov. 17, 1959; Freed U.S. Pat.No. 2,735,645, dated Feb. 21, I956; Freed U.S. Pat. No. 3,199,835, datedAug. 10, 1965; Dewrance, et al. U.S. Pat. No. 195,707, dated Oct. 2,1877; Schenck, et al. U.S. Pat. No. 2,987,295, dated June 6, 1961; andFloyd U.S. Pat. No. 3,090,594, dated Dec. 19, 1960.

Most, if not all, of the aforesaid patents disclose lined plug valveshaving a tapered plug in a lined valve body bore and which utilize anaxially shiftable pressure screw for elevating the plug towards itssmaller end for increasing the interference fit between the plug andliner, in an effort to overcome internal leakage, downstream past theplug; however, such tightening of the pressure screw and the resultantaxial movement of the plug has no beneficial or modifying effect uponthe top external seal which surrounds the actuating stem of the plug.

The balance of the relevant prior art is best exemplified in my U. S.Pat. No. 3,540,740 dated Nov. 17, 1970, which discloses means foreffecting a circumferential seal between the plug and valve body; and animproved seal between a portion of the top of the plug and a diaphragmengaged thereby, and an improved seal between the shank of the plug, adelta ring, and a pressure seal ring which is adjustably mounted formovement relative to the cover plate. The top or external seals of myaforesaid patent has been highly effective under most conditions,however, it has been found that in some instances the line pressureagainst the plug, when in closed condition, tends to push the plugagainst the downstream side of the valve bore for enhancing thecircumferential seal between the plug and valve body. Such movement ofthe plug, even though minute, sometimes results in a slight leak betweenthe plug and the primary circumferential seal on the upstream side ofthe valve. Such minute circumferential leakage past the body and theplug causes a pressure build-up on top of the plug which forces the plugdown with the result that the top or external seal between the plug andthe diaphragm, which it originally abutted, is impaired to such anextent that the top seal is rendered more or less ineffective andleakage occurs along the stem of the plug.

SUMMARY OF THE INVENTION The present invention involves the use of amicrometric or rather finely threaded adjustment screw advanceableaxially of the valve plug to compress the top and stem seal thereof.Equalized seating of the plug, and alignment ofits port with the portsof the valve body, are functions delegated to a three-point equalizingadjustment means surrounding the stem, as more fully disclosed in myU.S. Pat. No. 3,540,740. Motions of the three-point equalizingadjustment means and of the micrometric plug adjustment screw areinterrelated, in that an advancement of said screw to compress the topor stem seal produces in effect a practically imperceptible loosening ofthe plug relative to its tapered seat in the body bore; however, suchminute loosening of the plug is found not productive of leakage if theplug is otherwise properly adjusted, this being due to the memorycharacteristic of the plastic liner incorporated in the valveconstruction.

The invention also is very effective in preventing external leakage whenvalve is subjected to temperature changes. As valve is heated therelative thick section of Teflon surrounding the plug and lining thebody expands approximately 10 times the metal encapsulating body. Thiscauses Teflong to flow and results in a looser fit between body andplug. Plug can still seal downstream due to differential pressure andsince external seal means has considerable less thickness of Teflondifferential expansion does not effect the external seal.

An object of the invention is to provide in a lined plug valve, highlyeffective yet simple and inexpensive means for preventing both internaland external leakage of fluid-control valves, whether the valve is fullylined or only partly lined. The invention, moreover, is applicable toplug valves having right cylindrical as well as tapered plugs.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing, the referencenumeral 4 indicates a valve body, usually of cast metal, having a fluidinlet port 6 and a fluid outlet port 8, each of which ports may bedefined externally by suitable flanges 10, 10 or other means forconnecting the valve in a pipe system.

Between the ports 6 and 8, the body is transversely cored or bored toprovide a plug receptive cavity or bore 12 in which is supported anelongate rotary valve plug 14 having a through port 16, which uponrotation of the plug, may be placed in registry or out of registry withsaid ports 6 and 8 as is usual.

In the embodiment shown by way of example, lining elements 18 and 20line the body ports 6 and 8, respectively, as well as the plug receptivecavity 12. A lining element 22 may envelop the valving portion of theplug, including the port 16 and a portion of the actuating stem 24. Ifdesired, however, the plug may be formed without a lining element, inwhich case the material thereof shown cross-hatched for plastic on FIG.1, would in fact be metal.

The lining material shown at 18, and 22, may be a suitable plastic orfluorocarbon material having qualities such as will facilitate firmadherence to the metal surfaces in molding, along with imperviousnessand an inherently low coefficient of friction. Material such as Teflonmeets the requirements specified, and is further characterized by itshaving a constitutional memory, or ability to return substantially to aninitial state following a release from compression or other stresspreviously applied thereto. Due to differences in molding techniquespeculiar to coating of the plug and the valve body, ease of linerapplication may favor the use of different forms of plastic in coatingthese parts, if desired.

With further reference to the drawing, the plug is seen to have a stemportion 24, a circumferential end portion 26, and a circumferentialintermediate portion 28, said portions 26 and 28 flanking the plug port16. The outer areas of portions 26 and '28, and the ported area betweenthem, seat against the lining material of the body cavity that embracesthe plug.

The actuating stem 24 extends'axially from plug portion 28, and meetssaid portion 28 at an annular shoulder 30 which may be coated withlining material. Shoulder 30, by preference though not of necessity,lies in a plane which is substantially at right angles to the plug axis.it must be understood that shoulder 30, if

desired, may slope to some extent in the direction of circumferentialportion 28 without impairing the function of the shoulder, later to beexplained.

Stem 24 where engaged by the external seal elements 52, 54, 56, 62, maybe coated withthe plastic material of the plug as indicated at 34.Outwardly beyond said coating, that portion of the stem which is exposedmay be fitted with a hub 36 secured to the stem by means of a screw 38.A transversely directed handle 40 supported by hub 36 facilitatesrotation of the valve plug between open and closed positions.

v Cover 32 surrounds the stem and may be secured to the valve body by aplurality of bolts 44 carrying'nuts rounding the stem and underlying hub36, may serve as a dust guard for the concealed portion of the actuatingstem. The bonnet carries three adjusting screws 50, all of which areparallel to the plug axis and spaced equally therefrom, as well asequally from one another. The inner ends of the screws 50 bear upontheupper face of a rigid metal pressure ring 52 which loosely surroundsthe actuating stem.

The reference numerals 54 and 56 indicate two circular discs ordiaphragms each centrally apertured to receive the stem 24. The outer orcircumferential margins of diaphragms 54 and 56 are adapted to beclamped securelybetween bonnet 32 and an annular coated seat 58 providedon valve body 4. Seat 58 surrounds the actuating stem and pressure ring52, and may be countersunk in the valve body material as shown. Anannular lower clamping face 60 of thebonnet overlies seat 58 and theouter margins of diaphragms 54 and 56.

When nuts 46 are tightened, face 60 of the bonnet bears firmly againstthe diaphragms and coated annular seat 58 to form a leakproof sealbetween the clamped parts.

-46.- A centrally apertured coniform washer 48 sur- Diaphragm 56 may bea substantially flat disc of Teflon or equivalent fluorocarbon materialhaving a low coefficient of friction, and a marginal area of its centralaperture lies upon the exposed surface of coated plug shoulder 30. Theedge of the central aperture thereof preferably abuts or lay very closeto the upright cylindrical portion of stem 24.

Upon the marginal area of the central aperture of diaphragm 56 isdisposed a delta ring or similar deformable gasket 62, which surroundssnugly the coated portion of stem 24 closely adjacentto shoulder 30. Thedelta ring or gasket 62 may be formed of any suitable flexible materialwhich is capable of effecting a fluid seal along the coated stem, anexample of which is Teflon. The obliquely inclined outer-face of ring 62is exposed to a portion of the overlying diaphragm 54, and subtends theangle between coated should er 30 and coated stem 24.

Uniformly satisfactory results have been obtained in those instanceswhen diaphragm 54 which overlies diaphragm 56, is formed of afluorocarbon material such as Teflon wherein its central aperturemarginal area is inclined upwardly and inwardly as a continuous flange,to form an annular lip 64 inclined at an angle approximating the slopeof the outer face of ring 62. The innermost edge of flange or lip 64need not be in direct contact with stem 24.

Pressure ring 52 preferably has an obliquely inclined face or chamfer 66the angularity of which approximates the angular inclination of lip 64,so that force applied vertically downwardly upon ring 52 by screws 50,will serve to compact and expand the delta ring 62 by way of lip 64,into effective adjusted sealing contact I with the coated shoulder 30and coated stem 24.

- It may here be noted that the tripodal arrangement of adjusting screws50 makes possible a tilting of pressure ring 52, incident to the screwadjustments, whereby the valve plug may be cocked or biased in anydesired lateral direction within the plug receptive cavity 12. The plugthereby may be adjustably biased bodily toward body port 6 or towardbody port 8, as may be found effective to preclude internal leakage offluid past the plug under various service conditions which may take intoaccount high or low pressures or temperatures of fluid, or slightstructural variations in the valve manufacture or final assembly.

The plug receptive body cavity 12 initially is open at its lower end toexpose the coated lower face or terminal end portion 68 of the plug;however, in practice the lower end of the cavity is closed by a plate orcap 70 which may be detachably secured to the valve body as by means ofthreaded fasteners 72.

Cap 70 clamps and secures the outer marginal portion of a flexiblediaphragm 74 in leakproof manner against the valve body, or against theliner of the valve body at 76. The seal at 76 is annular and continuous,and will accordingly preclude all possibility of external leakage offluid past cap 70.

Cap 70 carries externally accessible means for bodily shifting the valveplug axially against the diaphragms 54, 56 and rings 62, 52, forperfecting-the actuating stem seal. Such means may include amicrometricadjusting screw 78 threadedly engaging internal fine threadsin a central bore 80 of the cap, and secura ble in adjusted position bya jam nut 82. The inner end of screw preferably supports a stool 84which does not rotate with the screw, this being to avoid chafing orpuncturing the flexible diaphragm 74 when screw 78 is rotated. Thediaphragm 74 preferably is formed of a flexible impermeable plastic orfluorocarbon material, of which Teflon is an example. Rubber orrubber-like diaphragms may be used in this valve if desired,this'depending upon the service conditions to which the valve may beexposed.

OPERATION OF THE DEVICE The valve assembled as shown may be connected ina pipe system for control of a liquid or gas under pressure. The fluidcontrolled is often highly corrosive, and may be conveyed through pipesand valves which are quite large. The largest valves may behydraulically or mechanically opened and closed, rather than manuallyactuated, particularly under high fluid pressure conditions.

initially, the tripodal screws 50 are adjusted so as to properly centeror locate the plug in the bore or cavity 12, with plug port 16 disposedin substantial registry with body ports 6 and 8. During which time theadjusting screw 78 is in a retracted position for providing a spacebetween the lower or terminal face of the plug and the adjacent surfaceof diaphragm 74. Adjustment of screws 50 also provides an effective topor external seal between the adjacent surfaces of upper shoulder 30 ofthe plug and diaphragm 56; between diaphragm 56 and 54; and between thedelta-ring 62, shank 34, diaphragm 56 and diaphragm 54. Thereafter theadjusting screw is advanced for applying a positive pressure against thelower surface of the plug via stool 84 and bottom diaphragm 74, asclearly illustrated in the drawing, for augmenting and enhancing the topor external seals between the plug and valve body.

When connected in a pipe system, the valve plug in closed position maybe subjected to upstream fluid pressure sufficient to cock or force theplug out of complete circumferential upstream contact with bore 12,resulting in circumferential internal leakage of fluid past the plug andthence upwardly into area J. Pressure in this area tends to displace theplug toward its lower end with concurrent impairment of the top sealbetween the plug and diaphragm 56 causing external leakage along theactuating stem 24. Such external leakage may be promptly and effectivelyovercome and terminated by advancing the micrometric adjustment screw 78in the direction of pressure ring 52, thereby to microscopically elevatethe plug to produce the desired compression between shoulder 30 of theplug, the diaphragm 54 and 56 and the delta ring 62, for therebyrestoring the top or external seal between the plug, stem and sealingelements associated therewith.

The extent to which the adjusting screw 78 need be advanced in order toterminate external leakage about stem 24 is infinitestimal, hence theuse of micrometric threads on the screw. The aforesaid adjustment atscrew 78 has no detrimental effect on the line sea] by reason ofupstream pressure supplementing line seal and the memory characteristicof the lining material which covers the plug and lines the bore orcavity 12 in which the plug rotates. 1

The present invention is also readily applicable to right cylindricalplug valves having a top or external seal as that term is used in thisdisclosure.

The present invention is also applicable to plug valves of the type inwhich uncoated metal plugs are received within a sleeve member offluorocarbon material, or the like, secured to and carried by the boreof a valve body, as in U.S. Pat. No. 3,540,740, so long as sealing meansare provided which engage the top, or portions of the upper surface ofthe plug as contrasted with sealing means which engage only the stemportion of the plug of a valve.

It should be understood that the drawing illustrates but one type of topor external seal, and that the subject invention is ideally adapted foruse with valves having other types of top seals, such as, by way ofexample, are illustrated in my U.S. Pat. No. 3,540,740.

What is claimed is:

l. A plug valve comprising, a valve body having inlet and outlet portstherein, a valve chamber in said valve body between said inlet andoutlet ports and intersecting said ports and open at its opposite endsthrough said valve body, a tapered valve plug rotatably mounted in saidchamber and having a transverse bore therethrough alignable with saidinlet and outlet ports, said tapered valve plug having an end face atone end thereof and extending at said one end into one of said chamberopen ends, the valve plug being larger in diameter at the other endthereof and extending at said other end into the other of said chamberopen ends, a reduced diameter valve stem on said other end of said valveplug and extending axially therefrom through said other open end of saidchamber to exteriorly of the valve body and defining with said valveplug an annular shoulder, external stem sealing means at said other openend of said chamber in sealing relationship with said valve stem, avalve bonnet secured to said valve body over said other open end of saidchamber and in surrounding relationship to said stem, an annularpressure ring in overlying relationship to the stem sealing means, aplurality of equally spaced adjusting screws extended through saidbonnet and into engagement with said pressure ring for pressing the stemsealing means into sealing engagement with said stem and to initiallyadjust the position of' said valve plug in said chamber, sealing meansat said one end of said valve plug and said one open end of said chamberto seal said one end of said valve plug relative to said valve body, acap secured to said valve body over said one open end, and an adjustingscrew means threadedly extended through said cap and into cooperativeassociation with said end face to urge said valve plug toward said stemsealing means to effect a leak tight seal at the other end of the valveplug.

2. A valve as defined by claim 1, wherein said adjusting screw meansincludes a rotatable micrometric screw having an outer end exposed formanipulation exteriorly of the valve body.

3. A valve as called for in claim 1, in which a flexible sealingdiaphragm is interposed between the end face of the valve plug and thesaid adjusting screw means for applying an axial force to said end faceof the plug.

4. A valve as called for in claim 3, wherein an enlarged bearing memberis interposed between said diaphragm and the said adjusting screw meansfor applying an axial force to the end face of the plug.

5. The valve as, defined by claim 1, wherein the rotatable screw hasmicrometric threads thereon for making fine adjustments of said plug.

6. A plug type valve comprising in combination:

a rotary plug which comprises, a circumferential end portion, acircumferential intermediate portion, an end face, and an actuatingstern having a free end extending from said intermediate portion; athrough port in the plug between the stem and said end face; an annularshoulder on said circumferential intermediate portion of the plug andsurrounding said stem, said shoulder having an annular surface exposedin the general direction of the free end of the actuating stern; a valvebody having 10 an inlet port and an outlet port, and a linedintermediate transverse bore rotatably supporting said circumferentialportions of the plug; a bonnet surrounding the actuating stem and havingan annular clamping face overlying an annular margin of said bore;diaphragm means having an aperture in which the stem is rotatable, saidaperture having an inner marginal portion overlying and contacting theexposed surface of said plug shoulder; said diaphragm means having anouter peripheral margin; means securing the bonnet upon the valve bodywith the outer peripheral margin of said diaphragm means clamped andsealed between said annular margin of the body bore and said clampingface of the bonnet; a pressure ring surrounding the stem, said ringhaving one face in contact with the diaphragm means, and an oppositeface exposed to the bonnet; a plurality of equally spaced adjustablemeans extended through said bonnet and into engagement with saidpressure ring for advancing the pressure ring against the diaphragmmeans and to force said diaphragm means against the aforesaid annularsurface of the plug shoulder; and adjustable means cooperativelyassociated with the end of the plug opposite said stem for bodilyshifting the plug axially in the direction of the bonnet, for modifyingthe force imposed by said plug shoulder surface and said pressure ringupon the inner marginal portion of the diaphragm means.

8. The valve as defined by claim 7, wherein the combination includes acap secured to the valve body in covering relation to said end face ofthe plug; and a flexible sealing diaphragm having a central portiondisposed intermediate said means at the inner end of said screw and saidplug end face; said last mentioned flexible sealing diaphragm having anouter marginal portion secured and sealed between the valve body andsaid cap.

9. The valve as defined by claim 7, wherein the combination includes acap secured to the valve body in covering relation to said end face ofthe plug; and a flexible sealing diaphragm having a central portiondisposed intermediate said means at the inner end of said screw and saidplug end face; said last mentioned flexible sealing diaphragm having anouter marginal portion secured and sealed between the valve body andsaid cap.

10. The valve as defined by claim 6, wherein the combination includes adeformable delta ring having sealing contact against the stem, andpositioned for wedging between the pressure ring and said diaphragmmeans first mentioned.

11. The valve as defined by claim 10, wherein the stem where contactedby said delta ring, and the plug in the regions of said shoulder, saidend face and said circumferential portions, carry a coating offluorocarbon material of the class of Teflon.

12. The valve as defined by claim 6, wherein the stem, and the plug inthe regions of said shoulder, said end face, and said circumferentialportions, carry a coating of fluorocarbon material.

13. The valve as defined by claim 6, wherein the aforesaid plurality ofadjustable means for advancing the pressure ring, compn'ses a set ofthree adjustable screws carried by the bonnet, said screws beingsubstantially equidistant from the plug stem, and substantiallyequidistant from one another.

14. The valve as defined by claim 6, wherein the aforesaid plurality ofadjustable means for advancing the pressure ring, includes means to tiltsaid pressure ring relative to the mean plane of the bonnet; and thestem and the plug in the regions of said shoulder, said end face, andsaid circumferential portions, carry a coating of fluorocarbon material.

15. The valve as defined by claim 14, wherein the combination includes adeformable delta ring having sealing contact against the stem, andpositioned for wedging between the pressure ring and said diaphragmmeans first mentioned.

16. The valve as defined by claim 6, wherein is included in thecombination a dust guard surrounding the actuating stem exteriorly ofthe bonnet.

1. A plug valve comprising, a valve body having inlet and outlet portstherein, a valve chamber in said valve body between said inlet andoutlet ports and intersecting said ports and open at its opposite endsthrough said valve body, a tapered valve plug rotatably mounted in saidchamber and having a transverse bore therethrough alignable with saidinlet and outlet ports, said tapered valve plug having an end face atone end thereof and extending at said one end into one of said chamberopen ends, the valve plug being larger in diameter at the other endthereof and extending at said other end into the other of said chamberopen ends, a reduced diameter valve stem on said other end of said valveplug and extending axially therefrom through said other open end of saidchamber to exteriorly of the valve body and defining with said valveplug an annular shoulder, external stem sealing means at said other openend of said chamber in sealing relationship with said valve stem, avalve bonnet secured to said valve body over said other open end of saidchamber and in surrounding relationship to said stem, an annularpressure ring in overlying relationship to the stem sealing means, aplurality of equally spaced adjusting screws extended through saidbonnet and into engagement with said pressure ring for pressing the stemsealing means into sealing engagement with said stem and to initiallyadjust the position of said valve plug in said chamber, sealing means atsaid one end of said valve plug and said one open end of said chamber toseal said one end of said valve plug relative to said valve body, a capsecured to said valve body Over said one open end, and an adjustingscrew means threadedly extended through said cap and into cooperativeassociation with said end face to urge said valve plug toward said stemsealing means to effect a leak tight seal at the other end of the valveplug.
 2. A valve as defined by claim 1, wherein said adjusting screwmeans includes a rotatable micrometric screw having an outer end exposedfor manipulation exteriorly of the valve body.
 3. A valve as called forin claim 1, in which a flexible sealing diaphragm is interposed betweenthe end face of the valve plug and the said adjusting screw means forapplying an axial force to said end face of the plug.
 4. A valve ascalled for in claim 3, wherein an enlarged bearing member is interposedbetween said diaphragm and the said adjusting screw means for applyingan axial force to the end face of the plug.
 5. The valve as defined byclaim 1, wherein the rotatable screw has micrometric threads thereon formaking fine adjustments of said plug.
 6. A plug type valve comprising incombination: a rotary plug which comprises, a circumferential endportion, a circumferential intermediate portion, an end face, and anactuating stem having a free end extending from said intermediateportion; a through port in the plug between the stem and said end face;an annular shoulder on said circumferential intermediate portion of theplug and surrounding said stem, said shoulder having an annular surfaceexposed in the general direction of the free end of the actuating stem;a valve body having an inlet port and an outlet port, and a linedintermediate transverse bore rotatably supporting said circumferentialportions of the plug; a bonnet surrounding the actuating stem and havingan annular clamping face overlying an annular margin of said bore;diaphragm means having an aperture in which the stem is rotatable, saidaperture having an inner marginal portion overlying and contacting theexposed surface of said plug shoulder; said diaphragm means having anouter peripheral margin; means securing the bonnet upon the valve bodywith the outer peripheral margin of said diaphragm means clamped andsealed between said annular margin of the body bore and said clampingface of the bonnet; a pressure ring surrounding the stem, said ringhaving one face in contact with the diaphragm means, and an oppositeface exposed to the bonnet; a plurality of equally spaced adjustablemeans extended through said bonnet and into engagement with saidpressure ring for advancing the pressure ring against the diaphragmmeans and to force said diaphragm means against the aforesaid annularsurface of the plug shoulder; and adjustable means cooperativelyassociated with the end of the plug opposite said stem for bodilyshifting the plug axially in the direction of the bonnet, for modifyingthe force imposed by said plug shoulder surface and said pressure ringupon the inner marginal portion of the diaphragm means.
 7. The valve asdefined by claim 6, wherein said adjustable means last mentionedincludes a rotatable screw having an outer end exposed for manipulationexteriorly of the valve body, and means at the inner end of said screwfor applying force to said end face of the plug.
 8. The valve as definedby claim 7, wherein the combination includes a cap secured to the valvebody in covering relation to said end face of the plug; and a flexiblesealing diaphragm having a central portion disposed intermediate saidmeans at the inner end of said screw and said plug end face; said lastmentioned flexible sealing diaphragm having an outer marginal portionsecured and sealed between the valve body and said cap.
 9. The valve asdefined by claim 7, wherein the combination includes a cap secured tothe valve body in covering relation to said end face of the plug; and aflexible sealing diaphragm having a central portion disposedintermediate said means at the inner end of said screw and said plug endface; said last mentioned flexible sealing diaphragm hAving an outermarginal portion secured and sealed between the valve body and said cap.10. The valve as defined by claim 6, wherein the combination includes adeformable delta ring having sealing contact against the stem, andpositioned for wedging between the pressure ring and said diaphragmmeans first mentioned.
 11. The valve as defined by claim 10, wherein thestem where contacted by said delta ring, and the plug in the regions ofsaid shoulder, said end face and said circumferential portions, carry acoating of fluorocarbon material of the class of Teflon.
 12. The valveas defined by claim 6, wherein the stem, and the plug in the regions ofsaid shoulder, said end face, and said circumferential portions, carry acoating of fluorocarbon material.
 13. The valve as defined by claim 6,wherein the aforesaid plurality of adjustable means for advancing thepressure ring, comprises a set of three adjustable screws carried by thebonnet, said screws being substantially equidistant from the plug stem,and substantially equidistant from one another.
 14. The valve as definedby claim 6, wherein the aforesaid plurality of adjustable means foradvancing the pressure ring, includes means to tilt said pressure ringrelative to the mean plane of the bonnet; and the stem and the plug inthe regions of said shoulder, said end face, and said circumferentialportions, carry a coating of fluorocarbon material.
 15. The valve asdefined by claim 14, wherein the combination includes a deformable deltaring having sealing contact against the stem, and positioned for wedgingbetween the pressure ring and said diaphragm means first mentioned. 16.The valve as defined by claim 6, wherein is included in the combinationa dust guard surrounding the actuating stem exteriorly of the bonnet.